1. Field of the Invention
The present invention relates to a combustion apparatus, and more particularly to a combustion apparatus utilizing swirling or cyclonic combustion for high specific heat release while producing exhaust gases with ultra-low concentrations of nitrogen oxide, commonly known as NO.sub.x.
2. Description of the Related Art
In the past, cyclone combustion chambers have been used to produce a cyclone of turbulent gases within a combustion chamber for combusting various solid materials, including poor quality coal and vegetable refuse. Combustors of this type are disclosed in "Combustion and Swirling Flows: A Review," N. Syred and J. M. Beer, Combustion and Flame, Volume 23, pages 143-201 (1974). This document is incorporated herein by reference. A firetube boiler having a cyclonic combustor was commercially marketed by Cyclotherm Division, Oswego Package Boiler Co., Inc.
Although known adiabatic cyclone combustors provide high specific heat release, such known combustors have the disadvantage that combustion temperature and NO.sub.x emissions are high. Moreover, in conventional cyclone combustors, combustion is unstable at low capacity burning and high turndown ratios are not possible in non-adiabatic combustors. The turndown ratio of a combustion apparatus in a boiler, defined as the ratio of maximum load to minimum load, measures the ability of the boiler to operate over the extremes of its load ranges. A high turndown ratio allows for a wide range in the level of steam generation at a particular time. This wide range is important to allow the boiler to efficiently respond to varying steam demands.
With pollution control requirements becoming constantly more stringent, it is necessary to reduce NO.sub.x emissions in exhaust gases while maintaining a high turndown ratio. For instance, the South Coast Air Quality Management District has proposed emission regulations for boilers, steam generators and process heaters that would require any units with capacities greater than or equal to 5 MM BTU/hour to limit their discharge of NO.sub.x to 40 PPM and carbon monoxide to 400 PPM at 3 percent oxygen. Even more stringent NO.sub.x emission standards are expected to be proposed and promulgated in the near future. It is expected that in certain parts of the country, NO.sub.x emissions will be required to be less than 25 PPM.
With pollution control requirements becoming increasingly more stringent, it is necessary to decrease NO.sub.x emissions even further than is achieved with presently known combustion apparatuses, while not increasing or while even decreasing the cost of the combustion equipment. A known technique of staged combustion, wherein primary air or initial combustion is at fuel rich conditions and secondary combustion air is added or staged to complete combustion, has been utilized in various combustion apparatuses to reduce NO.sub.x formation. It has previously been thought impractical to utilize staged combustion in a firetube boiler because there is no known way for introducing a secondary stream of air into the Morison tube. The Morison tube and outer shell of a firetube boiler comprise a pressure vessel which cannot be penetrated to introduce a second stream of air.
U.S. Pat. No. 4,565,137 to Wright discloses an apparatus for supplying four streams of air into a cyclonic combustion vessel connected to a firetube boiler for staged combustion of biomass materials. However, the disclosed apparatus is concerned with providing an increased rate of heat transfer from the flame to the Morison tube and minimizing slag formation in the combustion process. There is no disclosure in Wright of reducing NO.sub.x emissions to an ultra-low level, i.e., less than 25 PPM. To the contrary, the disclosed apparatus results in a high concentration of NO.sub.x in the exhaust gases. Wright increases the luminosity and emissivity of the flame to achieve an increased heat transfer rate. This increase in the flame temperature actually increases NO.sub.x emissions. Wright cannot produce the ultra-low NO.sub.x emissions that are obtained by the present invention.
There are approximately 250,000 existing firetube boilers in the United States and abroad. In their existing condition, these boilers are unable to meet the more stringent pollution control requirements currently being proposed. Therefore, in addition to the need for an apparatus to be used on new firetube boilers for reducing NO.sub.x emissions, a need exists for an apparatus which can be retrofitted onto existing firetube boilers for lowering their NO.sub.x emissions. In addition to the problem with providing staged combustion in a firetube boiler previously discussed, i.e., there is no known way for introducing a second stream of air into the Morison tube, other problems will be encountered in attempting to retrofit existing firetube boilers with an apparatus for staged combustion. For instance, the apparatus must be able to be fit onto the front flange of the boiler without cutting or welding any part of the pressure vessel, i.e., the Morison tube or the shell.